//! @file
//!
//! Copyright (c) Memfault, Inc.
//! See LICENSE for details
//!
//! Note that the CRC implementation is CRC-16/XMODEM, not CRC-16/CCITT
//! (canonically named "CRC-16/KERMIT").
//!
//! It implements the following polynomial:
//!   x^16 + x^12 + x^5 + 1 (0x1021)
//! More details:
//!   http://reveng.sourceforge.net/crc-catalogue/16.htm#crc.cat.crc-16-xmodem

#include <inttypes.h>
#include <stddef.h>

#include "memfault/config.h"
#include "memfault/core/compiler.h"
#include "memfault/core/math.h"
#include "memfault/util/crc16.h"

//! Users can provide their own implementation of memfault_crc16_compute by
//! setting MEMFAULT_CRC16_BUILTIN to 0 in their platform configuration file.
#if MEMFAULT_CRC16_BUILTIN

  #if MEMFAULT_CRC16_LOOKUP_TABLE_ENABLE

static const uint16_t s_crc16_table[] = {
  0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50a5, 0x60c6, 0x70e7, 0x8108, 0x9129, 0xa14a, 0xb16b,
  0xc18c, 0xd1ad, 0xe1ce, 0xf1ef, 0x1231, 0x0210, 0x3273, 0x2252, 0x52b5, 0x4294, 0x72f7, 0x62d6,
  0x9339, 0x8318, 0xb37b, 0xa35a, 0xd3bd, 0xc39c, 0xf3ff, 0xe3de, 0x2462, 0x3443, 0x0420, 0x1401,
  0x64e6, 0x74c7, 0x44a4, 0x5485, 0xa56a, 0xb54b, 0x8528, 0x9509, 0xe5ee, 0xf5cf, 0xc5ac, 0xd58d,
  0x3653, 0x2672, 0x1611, 0x0630, 0x76d7, 0x66f6, 0x5695, 0x46b4, 0xb75b, 0xa77a, 0x9719, 0x8738,
  0xf7df, 0xe7fe, 0xd79d, 0xc7bc, 0x48c4, 0x58e5, 0x6886, 0x78a7, 0x0840, 0x1861, 0x2802, 0x3823,
  0xc9cc, 0xd9ed, 0xe98e, 0xf9af, 0x8948, 0x9969, 0xa90a, 0xb92b, 0x5af5, 0x4ad4, 0x7ab7, 0x6a96,
  0x1a71, 0x0a50, 0x3a33, 0x2a12, 0xdbfd, 0xcbdc, 0xfbbf, 0xeb9e, 0x9b79, 0x8b58, 0xbb3b, 0xab1a,
  0x6ca6, 0x7c87, 0x4ce4, 0x5cc5, 0x2c22, 0x3c03, 0x0c60, 0x1c41, 0xedae, 0xfd8f, 0xcdec, 0xddcd,
  0xad2a, 0xbd0b, 0x8d68, 0x9d49, 0x7e97, 0x6eb6, 0x5ed5, 0x4ef4, 0x3e13, 0x2e32, 0x1e51, 0x0e70,
  0xff9f, 0xefbe, 0xdfdd, 0xcffc, 0xbf1b, 0xaf3a, 0x9f59, 0x8f78, 0x9188, 0x81a9, 0xb1ca, 0xa1eb,
  0xd10c, 0xc12d, 0xf14e, 0xe16f, 0x1080, 0x00a1, 0x30c2, 0x20e3, 0x5004, 0x4025, 0x7046, 0x6067,
  0x83b9, 0x9398, 0xa3fb, 0xb3da, 0xc33d, 0xd31c, 0xe37f, 0xf35e, 0x02b1, 0x1290, 0x22f3, 0x32d2,
  0x4235, 0x5214, 0x6277, 0x7256, 0xb5ea, 0xa5cb, 0x95a8, 0x8589, 0xf56e, 0xe54f, 0xd52c, 0xc50d,
  0x34e2, 0x24c3, 0x14a0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405, 0xa7db, 0xb7fa, 0x8799, 0x97b8,
  0xe75f, 0xf77e, 0xc71d, 0xd73c, 0x26d3, 0x36f2, 0x0691, 0x16b0, 0x6657, 0x7676, 0x4615, 0x5634,
  0xd94c, 0xc96d, 0xf90e, 0xe92f, 0x99c8, 0x89e9, 0xb98a, 0xa9ab, 0x5844, 0x4865, 0x7806, 0x6827,
  0x18c0, 0x08e1, 0x3882, 0x28a3, 0xcb7d, 0xdb5c, 0xeb3f, 0xfb1e, 0x8bf9, 0x9bd8, 0xabbb, 0xbb9a,
  0x4a75, 0x5a54, 0x6a37, 0x7a16, 0x0af1, 0x1ad0, 0x2ab3, 0x3a92, 0xfd2e, 0xed0f, 0xdd6c, 0xcd4d,
  0xbdaa, 0xad8b, 0x9de8, 0x8dc9, 0x7c26, 0x6c07, 0x5c64, 0x4c45, 0x3ca2, 0x2c83, 0x1ce0, 0x0cc1,
  0xef1f, 0xff3e, 0xcf5d, 0xdf7c, 0xaf9b, 0xbfba, 0x8fd9, 0x9ff8, 0x6e17, 0x7e36, 0x4e55, 0x5e74,
  0x2e93, 0x3eb2, 0x0ed1, 0x1ef0,
};
MEMFAULT_STATIC_ASSERT(MEMFAULT_ARRAY_SIZE(s_crc16_table) == 256,
                       "Unexpected CRC lookup table size");

uint16_t memfault_crc16_compute(uint16_t crc_initial_value, const void *data,
                                size_t data_len_bytes) {
  uint16_t crc = crc_initial_value;

  const uint8_t *curr_ptr = data;
  for (size_t i = 0; i < data_len_bytes; i++) {
    uint16_t table_idx = ((crc >> 8) ^ *curr_ptr++) & 0xff;

    crc = ((uint16_t)(crc << 8)) ^ s_crc16_table[table_idx];
  }

  return crc;
}

  #else  // MEMFAULT_CRC16_LOOKUP_TABLE_ENABLE

uint16_t memfault_crc16_compute(uint16_t crc_initial_value, const void *data,
                                size_t data_len_bytes) {
  const uint32_t polynomial = 0x1021;
  uint32_t crc = crc_initial_value;
  const uint8_t *curr_ptr = data;
  for (size_t i = 0; i < data_len_bytes; i++) {
    crc = (crc ^ (uint32_t)(*curr_ptr++ << 8));
    for (int bit = 0; bit < 8; bit++) {
      crc = (crc << 1);
      if (crc & 0x10000) {
        crc = ((crc ^ polynomial) & 0xFFFF);
      }
    }
  }
  return (uint16_t)crc;
}

  #endif  // MEMFAULT_CRC16_LOOKUP_TABLE_ENABLE
#endif    // MEMFAULT_CRC16_BUILTIN
